RU2751989C1 - Method and system for protection of optical communication channel in space optical communication systems from flashing with point and extended light sources - Google Patents

Abstract

FIELD: optics.SUBSTANCE: method is comprised of stages wherein an optical laser communication channel is installed, the optical emission is focused by a receiving telescope covered with a highly selective frequency filter with flexible panels of adjustable curvature installed thereon by means of hinge joints controlled by feedback signals from a CCD matrix, the focused emission is divided by a beam splitter into two beams: to the modulator and to the highly sensitive CCD matrix, one pixel of the modulator corresponds with each pixel or group of pixels of the CCD matrix, the signal from the pixels of the CCD matrix is directed to the spatial selection control unit setting the voltage at the pixels of the modulator. If no voltage is applied to the pixels of the modulator, all the radiation reflected from the modulator reaches the detector of the optical communication channel, if the maximum possible voltage is applied to the pixels of the modulator, the radiation therefrom does not reach the detector, if the illumination level at a certain pixel of the CCD matrix exceeds the specified level, a high voltage is applied to the corresponding pixel of the modulator and the radiation therefrom does not reach the detector.EFFECT: ensured possibility of suppressing flashing without introducing stricter requirements for angular accuracy of the guidance and containment system of the optical communication channel.10 cl, 3 dwg

Description

The technical field to which the invention relates

The claimed invention relates to methods for protecting a detector in space optical communication systems from exposure to point and extended light sources.

The main sources of flare are believed to be:

1. Point light sources - Sun, Moon, stars and planets

2. Extended light sources - the glow of the Earth's atmosphere

It also relates to an optical detector protection system that implements this method.

The invention is applicable, in particular, in the field of space optical communication between two spacecraft in low earth orbit.

State of the art

Methods for protecting an optical detector from exposure to light from extended light sources are known from the prior art.

Thus, patent RU2093872C1 describes a light-shielding device for space telescopes and spacecraft observation equipment from sunlight exposure and to protect individual structural elements.

The light-shielding device of space telescopes contains a tube and a screen connected to the drive, while the drive is made of two-degree with axes of rotation perpendicular to each other and the longitudinal axis of the tube, and the screen is connected to the drive by means of a remote mechanism, which can be made in the form of a sliding rod or in the form of several articulated links.

This method has the following disadvantages:

- Since the tube has a cylindrical shape, radiation can be scattered from its lateral surface into the pupil of the receiving telescope.

Methods for protecting an optical detector from exposure to point light sources are known in the art.

A system for receiving a light beam US9689667B2, containing an array detector, is known from the prior art. The system also contains a micromirror modulator, the radiation from which is reflected to the array detector. The radiation is filtered by a micromirror modulator, which is controlled by an electrical signal from a radiation source in real time.

This system has the following disadvantages in relation to the tasks of space optical communication:

- The micromirror modulator is controlled by an electrical signal from the radiation source. In space communication systems, the emitter is significantly removed from the receiver, so the electrical signal from it cannot be used directly.

Thus, the utility model RU169980U1 describes a matrix optical radiation flux attenuator designed to exclude background illumination of a photosensitive surface due to a powerful local attenuation of the optical radiation flux from high-intensity point light sources.

The matrix attenuator of the optical radiation flux contains an input lens, a matrix optical shutter, a focusing optical system, an optical radiation receiver, and a control circuit. A prism unit made in the form of a total internal reflection prism and a compensation prism is installed between the input lens and the matrix optical shutter. In this case, the matrix optical shutter is made in the form of a digital micro-mirror device with a matrix element in the form of a micro-mirror, and the number of matrix elements corresponds to the number of matrix elements of the optical radiation receiver.

This utility model has the following disadvantages in relation to the tasks of space optical communication:

- The performance of the CCD matrix is not sufficient for organizing a high-speed communication channel, and the use of higher-speed matrix detectors with spatial resolution is not justified, since it is excessive for the tasks of filtering background noise and creates an excessive load on electronic image processing systems.

Disclosure of invention

The technical problem of the claimed solution is to improve the reliability of the protection of the detector from illumination by point and extended light sources.

The technical result consists in providing the possibility of suppressing illumination without stricter requirements for the angular accuracy of the guidance and retention system of the optical communication channel.

The specified technical result is achieved in the method of protecting the detector of the receiving device of optical space communication systems from parasitic illumination by point and extended light sources and contains the stages at which: optical radiation is focused by the receiving telescope with flexible panels of adjustable curvature installed on it by means of hinged mountings, focused radiation divided by a beam splitter into two beams: one of them, passing further along the axis of the telescope, is transmitted to the modulator, the other, reflected at an angle to the axis, enters a highly sensitive CCD matrix; setting the correspondence between the pixels of the CCD matrix and the modulator so that each pixel or group of pixels of the CCD matrix corresponds to one pixel of the modulator; the signal from the pixels of the CCD matrix is directed to the spatial selection control unit, which sets the voltage on the pixels of the modulator; installing the detector of the optical communication channel in such a way that if no voltage is applied to the pixels of the modulator, all the radiation reflected from the modulator hits the photosensitive area of the detector of the optical communication channel; if the maximum possible voltage is applied to the pixels of the modulator, then the radiation from them does not fall on the photosensitive area of the specified detector, while if the illumination level at a certain pixel or group of pixels of the CCD matrix exceeds the specified one, then a high voltage is applied to the corresponding pixel of the modulator, and the radiation from it does not hit the optical communication channel detector.

An additional feature is that the method comprises an additional stage, at which the areas of high illumination of the CCD matrix are monitored by computer methods and the area corresponding to the radiation of the transmitting laser of the optical communication system is eliminated.

An additional feature is that a micromirror modulator is used as a modulator.

An additional feature is that a liquid crystal spatial light modulator is used as a modulator.

An additional feature is that the movable flat plates of adjustable curvature are controlled by the feedback signal from the CCD matrix.

An additional feature is that the beam splitter divides the radiation into beams of unequal intensity.

An additional feature is that the beam splitter divides the radiation into beams of equal intensity.

The specified technical result is also achieved with the help of an optical detector protection system, which implements the claimed method, comprising: a focusing radiation receiving telescope (2) with a narrow-band interference filter mounted on the lens and flexible panels of adjustable curvature installed by means of hinges (1), a beam splitter (3) performing dividing the focused radiation into two beams, the CCD matrix (4), which receives the first radiation beam from the specified beam splitter (3), the spatial selection control unit (7), to which the signal from the CCD matrix (4) is transmitted, the modulator (6) receiving a control signal from the spatial selection control unit (7) and an optical detector of the optical communication channel (5), to which the second radiation beam is transmitted, transmitted from the specified beam splitter (3) and reflected from the modulator (6).

An additional feature is that a micromirror modulator is used as a modulator.

An additional feature is that the surface of the mirrors of the micromirror modulator is set so that it coincides with the focal plane of the telescope objective.

An additional feature is that a liquid crystal spatial light modulator is used instead of a micromirror modulator.

Implementation of the invention

The claimed invention is illustrated using the drawings:

Fig. 1 is a block diagram of the detector protection system, where:

1 - flexible panels of adjustable curvature,

2 - receiving telescope

3 - beam splitter

4 - CCD-matrix

5 - optical communication channel detector

6 - modulator

7 - spatial selection control unit

Fig. 2 is a diagram of the operation of the system for protecting the detector from illumination from the atmosphere.

Fig. 3 is a diagram of the operation of the detector protection system against illumination from the side of the Sun, Moon, stars and planets

The device is designed to combat flares from the Sun, Moon and stars when optical communication lines between spacecraft are operating. The device can also be used for atmospheric flights.

To suppress the illumination from an extended light source, the atmosphere, a structure of flexible panels with adjustable curvature is used, designed to form a geometric shadow on the telescope objective when it hits it scattered from the panels of radiation of minimum intensity (Fig. 2).

To suppress illumination from point light sources, the Sun, Moon, stars and planets, an optical scheme based on a micro-mirror light modulator is used for spatially selective suppression of radiation from such sources (Fig. 3).

Typically, frequency and angle filtering is used to suppress backlight from distant light sources. This invention makes it possible to improve the corner filtration system. Usually, angular filtering is carried out using an optical system that acts as a telescope, designed so that there is a focal plane inside the system, onto which images from distant luminous objects are focused in the form of small luminous points. Since the stars, the Sun and the Moon are at a great distance, their image in the focal plane will be point-like. This allows you to select with the help of a diaphragm laser radiation coming from the communication satellite. Indeed, in the focal plane of the optical system, radiation from distant objects that are located at different angles to the axis of the optical system will be focused to different places in the focal plane, depending on their angular position. Therefore, it becomes possible to place a diaphragm (a small hole) in the place of the focal plane, where the useful radiation passes, thereby blocking the access to the further part of the optical system for radiation from unwanted light sources. However, this method has a significant drawback - when the satellite and luminous objects move, the focal spots both from the laser source and from distant light sources are displaced along the focal plane, which inevitably leads to the requirement for rigid stabilization of the flying object in space and, as a consequence, a sharp increase in complexity and cost optical communication channel retention systems.

The proposed method and a system based on it makes it possible to neutralize the influence of vibrations and vibrations of the aircraft on the efficiency of suppressing illumination without stricter requirements for the angular accuracy of the guidance and retention system of the optical communication channel. For this purpose, an additional element is introduced into the device - a digital micromirror device (DMD), consisting of a matrix with a large number of micromirrors, each of which can be independently rotated by a command from a computer. The surface of the mirrors of the micromirror modulator is set so that it coincides with the focal plane of the lens. If the micromirrors are not rotated, then its mirror plane acts like an ordinary mirror. In this case, in some places of the mirror surface, luminous points will be visible, which are images of objects of illumination and the transmitter of the laser optical communication system. The computer determines the positions of these luminous points, generates commands for turning the micromirrors, according to which the micromirrors, which have been hit by the luminous points corresponding to the light sources, rotate, and the light radiation does not pass further to the detector of the communication system. If during the movement of the apparatus the luminous points from the objects of illumination are displaced, then the tracking system tracks their movements, the computer generates commands for turning other, already illuminated, micromirrors. These micromirrors are rotated, and the previously rotated ones return to their original position. Since this process is dynamic and occurs in real time, angular filtration is preserved when the orientation of the apparatus is changed.

The system of protection of the detector from illumination from the side of point light sources works as follows (figure 1).

Optical radiation is focused by a receiving telescope with flexible panels of adjustable curvature mounted on it by means of hinged mountings. Radiation in the optical system in front of the focal plane is divided: its main part passes through the beam splitter (3) and is focused in the focal plane; the remainder is focused on the auxiliary focal plane. This auxiliary focal plane accommodates the CCD (4) or any other array receiver.

The correspondence between the pixels of the CCD matrix and the modulator is set so that each pixel or group of pixels of the CCD matrix corresponds to one pixel of the modulator; the signal from the pixels of the CCD matrix is directed to the spatial selection control unit, which sets the voltage on the pixels of the modulator; installing the detector of the optical communication channel in such a way that if no voltage is applied to the pixels of the modulator, all the radiation reflected from the modulator hits the photosensitive area of the detector of the optical communication channel; if the maximum possible voltage is applied to the pixels of the modulator, then the radiation from them does not fall on the photosensitive area of the specified detector, while if the illumination level at a certain pixel or group of pixels of the CCD matrix exceeds the specified one, then a high voltage is applied to the corresponding pixel of the modulator, and the radiation from it does not hit the optical communication channel detector.

Since the distribution of images of light sources in the main and auxiliary focal planes are identical or similar, it is possible to determine which of the micromirrors of the micromirror modulator are illuminated from the image formed on the matrix receiver and issue the appropriate commands to rotate them. Thus, the proposed design and technical solution provides effective continuous suppression of illumination at any change in the relative position of the sources of illumination and the position of spacecraft providing optical communication.

As an example, consider the following case. Let the spacecraft (SC) be in the Earth's orbit and establish an optical communication channel with another SC. The receiving telescope of the spacecraft (2) is directed exactly to the source of the optical signal, and the direction to the Sun enters its field of view. The telescope is covered with a highly selective frequency filter that suppresses all wavelengths except for a narrow range of values near the carrier wavelength of the optical transmitter of the communication system. Then, in a conventional angular filtration diaphragm system, the radius of the diaphragm decreases until the direct solar radiation ceases to fall on the detector. This reduces the field of view of the telescope, increases the requirements for the accuracy of pointing the telescope to the source of the optical signal, thereby aggravating the effect of vibrations and spacecraft oscillations on the stability of the communication channel.

In the proposed invention, the area on which the radiation of the Sun is concentrated is excluded by turning the micromirrors corresponding to the pixels of the CCD matrix illuminated by the Sun (4). Namely, the spatial selection control unit (7) reads information about the illumination of each pixel of the CCD matrix (4) and turns to the off position those micromirrors of the modulator (6) that correspond to pixels with a high level of solar illumination. Thus, the field of view of the telescope does not decrease, and the required angular accuracy of pointing and holding the optical channel does not increase; thus, vibrations and vibrations of the spacecraft do not affect the stability of the communication channel more than in the absence of solar illumination.

There are several main sources of optical noise in near-Earth space: the Sun (noise intensity 0.193 mW / cm / nm, Taylor M. IKONOS planetary reflectance and mean solar exoatmospheric irradiance // Space Imaging Inc .: Thornton, Colorado. - 2005.) , Moon (noise intensity about 1e-6 mW / cm2 / nm Karp S. et al. (Ed.). Optical channels: fibers, clouds, water, and the atmosphere // Springer Science & Business Media. - 2013 .), Earth (noise of the order of 40% of the solar, see Nahar SN Solar Irradiance of the Earth's Atmosphere // Climate Change and Food Security in South Asia. - Springer, Dordrecht, 2010. - pp. 31-42.). The noise from the planets is weaker than the noise from the Moon by another 4 orders of magnitude, so it is not taken into account.

Let's calculate the maximum possible noise. The total noise intensity will be about 0.27 mW / cm2 / nm, which, when converted to the number of photons, is 4.3e15 1 / s / nm. Since we work at a frequency of 1 GHz, there will be 4.3e6 1 / nm / ns noise photons per clock cycle. When using our proposed spatial filtering system based on DMD, it is possible to reduce the noise power by at least 95% (cutting it off at a level of about 1% in power), taking into account the final pixel size of the CCD camera and DMD, that is, 2.17e5 1 / nm / ns, which may be included in the dynamic range of an optical communication detector (for example, a solid-state photomultiplier tube).

1/нс.We assume that the noise has a Gaussian power distribution, and it is possible to shift the noisy signal so that the mean of the noise is equal to 0, therefore, the actual amplitude of the noise per clock cycle can be considered the variance of its mean, namely

1 / ns.

Thus, the proposed filtering system can ensure the operation of the optical information receiver against the Sun, however, the error level in this operation requires a separate calculation. So, in the work (Peskov S.N., Ishchenko A.E. Calculation of the error probability in digital communication channels // Tele-satellite. - 2010. - No. 11. - P. 70.) provides a formula for the bit error (20 ), which for binary pulse encoding can be rewritten as follows:

— энергия сигнального импульса, а

— спектральная плотность шума. Так как полоса приемника ок. 1 нм, будем считать, что

— энергия шума на такт. Положим энергию сигнального импульса равной 1000 фот., а амплитуда шума равна 1.75е2 фот./с, то

, откудаHere

Is the signal pulse energy, and

Is the spectral density of the noise. Since the receiver bandwidth is approx. 1 nm, we will assume that

Is the noise energy per clock cycle. We put the signal pulse energy equal to 1000 photons, and the noise amplitude is equal to 1.75e2 phot / s, then

, where

Well-known error correction algorithms (Reed-Solomon codes, turbo codes, convolutional codes, etc. Sklar B. Reed-solomon codes - 2001. - P. 1-33.) Completely (up to BER ≤ 1e-20) restore information at physical channel error BER ≤ 1e-4.

Thus, to overcome the disadvantages of the analogues of the invention RU2093872C1, the claimed invention uses movable flexible plates with adjustable curvature on hinged mountings, controlled by feedback signals from the CCD matrix.

To overcome the disadvantage of the US9689667B2 system, a CCD array is used to provide feedback to control the micromirror modulator without a direct electrical connection to the radiation source.

To overcome the drawback of the RU169980U1 utility model, the radiation after the receiving telescope is divided into two beams, one of which forms an image on the CCD matrix, the signal from which is processed by the spatial selection control unit, which generates control signals for the micromirror modulator. Another beam is reflected from the micromirror modulator and absorbed by an optical communication detector designed to receive the information signal. Consequently, a low-speed CCD matrix is used to filter the radiation, and a high-speed optical communication detector is used to receive information.

Claims (16)

1. A method for protecting a detector of an optical communication channel of a receiving device of optical space communication systems from parasitic illumination by point and extended light sources, containing the stages at which establish an optical laser communication channel between spacecraft, one of which is in the Earth's orbit; the optical radiation is focused by a receiving telescope covered with a highly selective frequency filter, with flexible panels of adjustable curvature mounted on it by means of hinged mountings, controlled by feedback signals from the CCD matrix; the focused radiation is divided by a beam splitter into two beams: one of them, passing further along the axis of the telescope, is transmitted to the modulator, the other, being reflected at an angle to the axis, enters the highly sensitive CCD matrix; setting the correspondence between the pixels of the CCD matrix and the modulator so that each pixel or group of pixels of the CCD matrix corresponds to one pixel of the modulator; the signal from the pixels of the CCD matrix is directed to the spatial selection control unit, which sets the voltage on the pixels of the modulator; the detector of the optical communication channel is installed in such a way that if no voltage is applied to the pixels of the modulator, all the radiation reflected from the modulator falls on the photosensitive area of the detector of the optical communication channel; if the maximum possible voltage is applied to the pixels of the modulator, then the radiation from them does not fall on the photosensitive the area of the specified detector, in this case, if the illumination level at a certain pixel or group of pixels of the CCD matrix exceeds a given one, then a high voltage is applied to the corresponding pixel of the modulator, and the radiation from it does not fall on the detector of the optical communication channel. 2. The method according to claim. 1, characterized in that it contains an additional stage, which makes tracking the areas of high illumination of the CCD matrix by computer methods and excludes the area corresponding to the radiation of the transmitting laser of the optical communication system. 3. The method according to claim 1, characterized in that a micromirror modulator is used as the modulator. 4. The method according to claim 1, characterized in that a liquid crystal spatial light modulator is used as the modulator. 5. The method according to claim 1, or 2, or 3, characterized in that the beam splitter divides the radiation into beams of unequal intensity. 6. The method according to claim 1, or 2, or 3, characterized in that the beam splitter divides the radiation into beams of equal intensity. 7. The protection system of the detector of the optical communication channel, which implements the method according to claim 1, comprising a focusing radiation receiving telescope installed on the spacecraft, covered with a highly selective frequency filter that suppresses all wavelengths except for a narrow range of values near the carrier wavelength of the optical transmitter of the communication system with a narrow-band interference filter installed on the entrance pupil and flexible panels of adjustable curvature installed by means of hinges, controlled by feedback signals from the CCD matrix (1), a beam splitter (3), which separates the focused radiation into two beams, a CCD matrix (4), into which receives the first beam of radiation from the specified beam splitter (3), the spatial selection control unit (7), to which the signal from the CCD matrix (4) is transmitted, the modulator (6), receiving the control signal from the spatial selection control unit (7), and optical communication channel detector (5), to which in front of The second radiation beam is transmitted, transmitted from the specified beam splitter (3) and reflected from the modulator (6). 8. The system according to claim 7, characterized in that a micromirror modulator is used as the modulator. 9. The system of claim. 7, characterized in that the surface of the mirrors of the micromirror modulator is installed so that it coincides with the focal plane of the telescope objective. 10. The system of claim. 7, characterized in that instead of the micromirror modulator, a spatial light modulator based on liquid crystals is used.

Images